These amides belong to a very important class of chemicals having applications as intermediates for pharmaceuticals, azo and sulfur dyes, fine chemicals, drugs, as stabilizers for hydrogen peroxide, as photographic chemicals and as antioxidants. N-acetyl p-amino phenol (APAP), commonly known as acetaminophen or paracetamol, is known for a wide variety of uses. Its medicinal use is very well known, notably as a non-prescription analgesic and anti-pyretic agent with properties similar to aspirin. It is also a major component in over 200 other drug formulations.
Amide formation is generally performed with acyl chlorides or anhydrides in presence of bases, 4-dimethylaminopyridine, 4-pyrrolidinopyridine, trimethylamine etc. Lewis acids such as zinc chloride, cobalt chloride, scandium trifluoromethanesulphonate which are good in the acetylation of amines with acetic anhydride generates one molecule of carboxylic acid for every molecule of anhydride consumed and therefore renders the process very expensive. To optimise the yields of ester/amide formation 2-15 molar equivalents of condensing agents such as sulfuric acid, tosylchloride, trifluoroacetic anhydride, polyphosphate ester, N, N.sub.1 -dicyclohexylcarbodiimide, graphite etc., are generally employed. Heterogeneous acidic and superacid catalysts have proved to be useful in some reactions because of their activity, selectivity, reusability, non-corrosivity and virtual absence of effluent treatment which usually is associated with the homogeneous catalysts.
U.S. Pat. No. 2,462,221 Feb. 22, 1949 describes a process wherein acetanilide is prepared by heating aniline with 20% excess of acetic acid in the presence of 2-5% by wt of benzene. The reaction mixture was heated in the range of 130-200.degree. C. Toluidine is acetylated in similar way to get very good yields. U.S. Pat. No. 2,551,047 May 1, 1951 describes a process wherein N-aryl acetamides are prepared by heating an arylamine with up to 25% excess of acetic acid in presence of up to 5% of AcOR, which acts as entrainer of water. The reaction temperatures are in the range of 130-200.degree. C. and the reaction time is 21 hrs for acetanilide preparation. The drawbacks in the above-described processes are that reactions are of quite long duration and the reaction temperatures used are very high. U.S. Pat. No. 4,288,592 Sep. 8, 1981 describes a process wherein the amides are prepared by reacting amines with acid halides or anhydrides in an inert organic diluent, in the presence of molecular sieves. The drawbacks in the above-described processes are that acid halides or anhydrides are used as acylating agents and all the reactions are performed in nitrogen atmosphere.
A communication in J. Chem. Soc., Chem. Commun., 1987,114 discloses the acetylation of amines in presence of acetic anhydride as the acetylating agent and cobalt(II) chloride as catalyst. The drawbacks in the above process are in the use of expensive acetic anhydride.
A communication in J. Chem. Soc., Chem. Commun.,1997,1389, discloses the use of montmorillonite K-10 and KSF catalysts for the acetylation of primary and secondary amines in presence of acetic anhydride the acetylating agent and finds that the results are remarkable. The drawbacks in the above process are the use of an expensive acetic anhydride as acetylating agent. Also, the reactions are not selective i. e., both the functional groups are acetylated in the case of 2-aminophenol, depending upon the molar ratios of the substrate and the acetic anhydride used. Moderate yields are obtained in this process.
In almost all the U.S. Pat. Nos. 4,264,526; 4,264,525; 4,565,890; 3,076,030; 3,341,587 and 5,155,269, the acetylation of p-amino phenol was performed in presence of acetic anhydride/acetic anhydride in aqueous solvent system. U.S. Pat. No. 4,264,526, Apr. 28, 1981 describes a process for the production of aminophenols and N-acetyl p-aminophenol (APAP) comprising alkaline hydrolysis of halonitrobenzene to nitrophenol and from nitrophenols to aminophenols using a borate ion additive during hydrogenation to eliminate undesirable by-products and colour formation. CS Patent 223,945 (Cl. C07C 91/44) Nov. 15, 1985 discloses a process wherein the acetylation of aminophenols with acetic anhydride in ethyl acetate or AcOH gives moderate yields of acetaminophen. The drawbacks in the above processes are the use of acetic anhydride as acetylating agent and the yields are moderate.
U.S. Pat. No. 4,670,589, Jun. 2, 1987 describes a process for the production of APAP by hydrogenation of p-nitrophenol to p-amino phenol (PAP), and concurrently acetylating the PAP with acetic anhydride. U.S. Pat. No. 5,648,535, Jul. 15, 1997, describes a process for the production of N-acylaminophenols by the concurrent hydrogenation of a nitrophenol to an aminophenol and the acylation of the aminophenol with acyl anhydride on a continuous basis in a stirred tank reactor. The drawbacks in the above processes are the use of excess acetic anhydride as acetylating agent/solvent system, difficulty of mono-acetylating the amino group, longer reaction times, oligomerization of the hydroxyl aromatic amine, and color body formation.
Obviously different approaches have been employed both on laboratory and commercial scale to prepare the industrially useful amides, and the traditional homogeneous catalysed reactions are being less favoured owing to the problems of separation and reuse. The present trend is to develop solid acids from cheaply available sources, and especially of clays.